This invention relates to hydraulic dampers and, particularly to hydraulic dampers of the kind comprising a cylinder containing therein hydraulic fluid, a piston working in the cylinder and partitioning the interior thereof into two liquid chambers, a piston rod secured to the piston and extending through one of the liquid chambers to the outside, and two valve discs provided respectively on opposite surfaces of the piston for generating a damping force in the extension and contraction strokes of the damper respectively.
Conventionally, there is provided in the hydraulic damper of the kind aforementioned a fixed orifice for permanently connecting the two liquid chambers, thereby maintaining the valve discs in the closed condition when the reciprocating velocity of the piston is low. The orifice generates a predetermined damping force both in the extension and contraction strokes and, defines a rising portion in damping force-velocity characteristics of the damper. The effective area of the orifice is the same between the extension and contraction strokes of the damper, and thus it is not possible to change the damping force between the extension and contraction strokes when the piston velocity is low. However, when it is desired to change the damping force between the extension and contraction strokes, it is not sufficient to change only the characteristics of the valve discs, and it is necessary also to change the characteristics of the orifice between the extension and contraction strokes of the damper.
Japanese Utility Model Disclosure (Kokai) No. 55-82539 discloses a hydraulic damper of the kind aforementioned. An orifice and a non-return valve are provided in connection with each of the valve discs, and the non-return valve acts to prevent the fluid flow in the direction closing the corresponding valve disc. Thus, the effective area of respective orifices can be changed with respect to the fluid flow passing through the piston. The damper operates satisfactorily but the construction thereof is complicated.